(432d) Critical Assessment on Indirect Ocean Capture of CO2 for Desalination Brine Valorization

Given the concerns of global climate change, the concept of CO₂ removal from the atmosphere has attracted substantial interest. Among the various CO₂ removal technologies, indirect ocean capture (IOC) is an emerging approach for direct removal of atmospheric CO₂ via extraction of dissolved inorganic carbon (DIC) from seawater, followed by air-seawater re-equilibration. Unlike CO₂ capture from large stationary emission sources, IOC has the potential to manage both the existing excess CO₂ in the atmosphere, as well as CO₂ from mobile or smaller point sources. Moreover, given that the CO₂ concentration (mg/L) in seawater is approximately 140 times greater than that in air, IOC can avoid processing the large volumes of air (e.g., via pumping and compressing) that is required for Direct Air Capture (DAC) techniques. Motivated by the growing body of literature on IOC in the last 5 years, in this study, we present a critical review and provide a quantitative perspective on IOC technologies which then provides support for the proposed process scheme for integration of IOC with a seawater reverse osmosis (SWRO) desalination plant to extract CO₂ from RO brine. Techno-economic analysis (TEA) and life-cycle assessment (LCA) was then carried out to evaluate the mutual benefits to both the IOC facility and the desalination plant via.

Here we note that control of solution pH and CO₂ partial pressure has been employed in various proposed IOC technologies, including for example, membrane separation, ion-exchange resin, and electrochemical techniques. However, most of the studies are still at the bench-scale proof-of-concept stages and face various challenges that include: (i) insoluble scales form and deposit at the cathode and membrane surfaces, leading to increased cell resistance and compromised electrolytic efficiency, (ii) slow CO₂ extraction rate and thus low extraction efficiency, (iii) high electrochemical energy consumption, and (iv) high capital and operating costs due to membrane contactor vacuum stripping and seawater pretreatment. At the current state-of-the-art, bipolar membrane electrodialysis (BPCEM) has been touted as one of the most practical IOC methods (with up to 98% lower electrochemical energy consumption). The above technology was thus considered in analyzing the potential adoption of IOC via TEA and LCA.

TEA analysis suggests that a stand-alone IOC facility is not yet feasible. However, the overall IOC cost via co-location with other ocean water-pumping technologies, especially seawater desalination plants, can be reduced up to 59% compared to a stand-alone facility. The IOC cost can be further reduced by utilizing SWRO desalination brine, which will double the CO₂ concentration in the seawater; hence, a reduction in the volume of seawater that requires processing via IOC. More importantly, as CO₂ solubility decreases with increasing solution salinity, a much higher CO₂ extraction rate can be achieved by spontaneous degassing of the acidified RO desalination brine (73%) relative to acidified seawater (40%). Consequently, the proposed IOC process, co-located with a seawater desalination plant using RO brine for CO₂ extraction, can reduce the cost of IOC to $138/tCO₂, which is ca. 96% lower than a stand-alone IOC system. It is also stressed that IOC co-location is beneficial to the seawater desalination plant since it will lead to (i) generation of extra revenue through RO brine valorization, and (ii) lowering the total volume and salinity of the brine that needs to be discharged for disposal, ultimately mitigating the negative environmental impact of the desalination process. In addition, the carbon credit earned through CO₂ capture can provide extra revenue for the desalination industry, leading to a reduction of freshwater production cost by ~6% (using a projected carbon price of $150/tCO₂ in 2050). Finally, integrating IOC can help the desalination industry achieve negative carbon emissions. In conclusion, the IOC co-location with seawater desalination plants will have the benefits of economic CO₂ removal, combating global climate change together with other CO₂ removal technologies, and improving the sustainability and cost efficiency of seawater desalination. Implementing the above proposed approach will require research efforts that focus on the development and optimization of the IOC technologies.